In a large skillet, fry the bacon until crisp, reserving about 2 tablespoons of the drippings. Remove the bacon from the pan and sautÚ the mushrooms in the reserved bacon grease until tender and the liquid has evaporated. Season with a little salt and pepper. Spread the cheese in a greased 11.75x7.5" baking dish. Top with the mushrooms, then the bacon and green onions. In a medium bowl, beat the eggs, cream, salt, celery salt and a dash of pepper. Pour evenly over everything. Bake at 350║ 35-40 minutes, until a knife inserted in the center comes out clean. Let stand 10 minutes before serving.